Environmental chemistry has been defined as the study of the sources, reactions, transport, effects and fate of chemical species in the environment, taken to mean water, air and soil and living organisms.
Environmental chemistry investigates at different  levels (depending on the problem) the state of health of the environment, from microhabitats to more complex ecosystems to global compartments. This state of health, in turn, reflects the consequences of past behaviour. Environmental chemistry identifies and measures natural and manmade chemical species and tries to understand the fate (mobility, residence time, transformations) and effects of these on the environment. Analytical chemistry is at the basis of any environmental research, but, once the intended use of analytical data (site characterization, monitoring of compliance with regulations, determination of the degree of contamination, toxicological risk assessment, personnel monitoring, remediation studies) has been established, the essential tasks for the environmental chemist are: preparing sampling methods which meet the intended aim, selecting appropriate analytical methods, interpreting data and ensuring data validation for the purposes of legal defensibility.
The broad area of environmental chemistry encompasses analytical chemistry, organic and inorganic chemistry, radiation chemistry, chemical engineering, soil chemistry, chemical toxicology and statistics. A challenge and a need for the environmental chemist is represented by team work, working alongside all the users of  environmental data (ecologists, biologists, geologists, hydrogeologists, toxicologists, and environmental engineers), because only through such an interdisciplinary approach can we fully understand an environmental problem.

	Fig. 1: The biogeochemical cycle of mercury. Once released into the environment, mercury undergoes a complex series of transformations. In certain conditions the species methylmercury (CH3Hg+) is formed, which can act on organisms as a powerful neurotoxin. (Credit: Academy of Natural Sciences - Estuarine Research Center)

When we discover in an ecosystem elements critical for the health and survival of the organisms which live in it, it is likely that this is due to polluting chemical substances.  The relative study of environmental chemistry would consist of the following steps:

• Planning an appropriate sampling and data collection strategy.
• Identifying the nature of the pollution (qualitative analysis).
• Identifying the extent (aerial study) and the level of pollution (quantitative survey).
• Tracking the source of the pollution: isolated incident or  long-standing emission from a point-source, etc.
• Evaluating environmental mobility: how a substance can split and be transferred to the various environmental compartments (atmosphere, water, soil and sediment, living organisms).
• Assessing the most important transformations of the substance in the environment (hydrolysis, photolysis, oxidation, biodegradation) and the resultant average residence time in the various compartments.

Fig. 2: A typical food web. A pollutant which enters the land may undergo chemical transformations or microbiological transformations, and may enter the food web and undergo biomagnification. One example is DDT, a pesticide which persists in the environment for a long time and which, transported along the marine food web, has been detected in fat tissues of top polar carnivores. (Credit: Soil Biological Communities)

Fig. 3: The first Nobel Prizes for Environmental Chemistry. The ozone layer - the Achilles heel of the biosphere.